Beam expanders are used to enlarge the size of a beam of light. Beam expanders are often used in laser applications where a laser beam from a light source starts out having a very small diameter but needs to be expanded in at least one direction for use downstream. In many cases, the laser beam has a high intensity and needs to be expanded to avoid damaging the downstream optical components.
Some beam expanders are catoptric optical systems, i.e., all of the optical components are reflective. Reflective optical elements are often preferred for beam expanders that operate at multiple wavelengths because they do not suffer from chromatic aberration and therefore do not require color correction.
FIG. 1 is a plot of the reflectance R (%) versus wavelength λ (nm) for a diamond-turned and optically polished aluminum mirror having a mirror substrate made of aluminum alloy (Al 6061-T6). The mirror includes a conventional quarter-wavelength HfO2/SiO2 multilayer reflective coating optimized at λ=1064 nm. The plot shows a broadband spectral reflectance from the ultraviolet (UV) to the near infrared (NIR), with reflectances R of 92.3% at 355 nm, 92.0% at 532 nm and 95.0% at 1064 nm. The three wavelengths selected are the common output wavelengths for a high-power Nd:YAG laser. The reflectance R at 355 nm has a bandwidth of only 24 nm. In the visible spectral range, the average reflectance is similar to that of bare aluminum.
For a beam expander that uses two of the aluminum mirrors having the reflectance properties of FIG. 1, the optical transmittance is 85.2% at 355 nm, 84.6% at 532 nm and 90.3% at 1064 nm. There are three main drawbacks associated with such a multiband laser beam expander, namely, a relatively low optical transmittance, a low laser-damage resistance, and susceptibility to surface degradation over time when exposed to extreme environments.
A beam expander for use with high-power lasers needs to have much higher optical transmittance (e.g., >95%) at each of the designated (operating) wavelengths, and preferably is resistant to laser-damage resistance and relatively insusceptible to surface degradation.